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Ghosts under the earth

Detectors accompany the disappearance of the most abundant particles in the Universe

Some answers from physics could be responded to from underground: there one can find one of the 1,000 ton detector pieces

One needs to remain for three minutes inside an elevator and travel in total darkness down 710 meters to reach the bottom of an ancient iron ore mine in the central-east of the United States called Soudan, now transformed into a high energy physics laboratory. As soon as one leaves the elevator the main equipment can be seen to the left: a particle detector formed by 486 octagonal sheets of pure steel, aligned like the slices of a loaf of bread, each one measuring 7.6 meters. This almost 6 ton detector functions in harmony with another, a little smaller, with 282 steel sheets and weighing 1,000 tons some 750 km distance away and at a depth of 105 meters – both form one of the experiments of the Fermi National Accelerator Laboratory (Fermilab), which is located close to Chicago. The initial results of these experiments, gathered after a year of operation and examined by a group of physicists, including Brazilians, present crucial aspects on particle behavior without which the Sun could not shine: neutrinos. They are one of the most abundant components of the Universe – each cubic meter contains 1 billion neutrinos and only 1 proton, another type of particle of much greater mass – and still they are being formed all of the time in the Sun’s interior when the helium atoms fuse with those of hydrogen.

The detectors of Minos, an acronym for Main Injector Neutrino Oscillation Search, have proven that the neutrinos disappear, as experiments carried out in Japan had indicated, and went a little beyond, showing how they disappear. In a few months, when the analysis of the data has been completed, perhaps it will be possible to know into what other forms these electrically neutral particles can transform themselves. There are three types of neutrino, each one of them associated to an electrically charged particle: the muon neutrino, the tau neutrino and the electron neutrino. The preliminary results suggest that the muon neutrinos should convert themselves into tau neutrinos with a probability hundreds of times greater than transforming themselves into electron neutrinos, according to Carlos Escobar, a physics professor from the State University of Campinas (Unicamp) who was a member of the group analyzing the results, alongside Philippe Gouffon, from the University of Sao Paulo (USP). Also participating in the Minos  experiment  were physicists from France, the United States, Greece, the United Kingdom and Russia.

The muon neutrinos studied in this experiment were formed from a collision of protons with a graphite target in the interior of a tube of 1 kilometer extension, the NuMi, which stands for Main Injector Neutrino Oscillation Search.. They passed through a 200 meter rock barrier without any effort and found the detector closest to the surface. Passing through there were thousands of neutrinos, but the majority dispersed, journeying below the earth, and 2.5 milliseconds afterwards only 92 neutrinos reached the ancient iron ore mine. “They would reach 177 if there were no oscillation”, says Escobar (oscillation is the transformation of one type of neutrino into another). “If the neutrinos disappear”, adds physicist Gouffon, “the only explanation is that a change of identity had occurred”.

For the physicists, this phenomenon is a clear demonstration of the mass of neutrinos, about which until recently there has still been doubt, since the particles of one type can only convert themselves into another if they present masses and different energies. Experiments done last year in Canada and in Japan, had demonstrated that the neutrinos have a mass 500,000 times less than that of the electron. The mass of these particles and the metamorphoses through which they pass, could be linked to the origin of protons, of electrons and of all the other fundamental elements of material. Indeed, “the mass of the neutrino could even explain our existence”, comments Hitoshi Murayama, a physicist from the University of California in Berkeley, the United States, in an article in the magazine, Physics World.

“It’s very good to see that the MINOS experiment is already producing important results, in spite of only having been in operation a year”, says Píer Oddone, the Fermilab’s director, on announcing these discoveries, on the afternoon of March 30th. “The MINOS results, with only a year of operation, waters one’s mouth”, comments Escobar. But the satisfaction adds to a good dose of concern: there is no guarantee that the complaints of the specialists in neutrinos will be attended to so that a 4 megawatt particle beam will be installed at the MINOS, seen as indispensable to allow for real advances in research (the current experiments were done using a beam of 140 kilowatts).
Other Fermilab teams are living through the prospect of budget cuts, the reason being the United States interest in participating, in the best manner possible, in the project for a super-accelerator, the International Linear Collider (ILC). Escobar knows, indeed, that, once the choice is made, the 2,300 people who work at the Fermilab are going once again to group themselves around common objectives. “At the Fermilab there’s competition between groups”, he says, “but not fratricidal fighting as in Brazil”.